The invention relates generally to a reusable ink sheet for transferring information onto a recording medium and more particularly to a method and apparatus for replenishing the ink that is transferred from the ink sheet during printing.
Conventional ink sheet printing methods have drawbacks. Once a dot of ink is transferred from the ink sheet to a transfer medium, that portion of the ink sheet is no longer useable. Consequently, printing methods that utilize conventional ink transfer sheets are expensive and wasteful because of the need to continuously replace partially depleted ink transfer sheets.
A method for recoating the ink layer of an ink sheet is discussed in "A Color Thermal Transfer Printer With Recording Mechanism", SID '85 Digest, pp. 143-45. This method essentially involves providing an ink sheet in the form of an endless belt and continuously replacing the entire ink layer with fresh, melted ink from a storage container. The ink is allowed to cool and the replaced ink layer is ready for printing.
This method has certain disadvantages. Considerable time is necessary for a warm-up while the ink in the storage container is melted. Further, a large amount of electricity is needed to maintain the replacement ink in a molten state. This method is inherently inefficient in that rather then replenishing only selected portions from which ink had been transferred the entire ink layer is continuously replaced.
Apparatuses for performing this method can be expensive and complicated. Mechanisms are needed to insure that the replenished ink layer is of uniform thickness. Additional mechanisms are required to eliminate molten ink from the ink sheet after the printer is turned off. Such a apparatus would be large and require considerable maintenance for proper continued operation.
A method of replenishing an ink sheet that uses powdered ink is described in the U.S. Pat. No. 4,467,332 in which powdered ink is transferred from the surface of an electrode mainly to depleted recorded portions of the ink sheet. This method also has shortcomings. Powdered ink unintentionally adheres to unused portions of the ink sheet. Although the amount of ink adhering to the recorded and unrecorded portions of the ink layer can be controlled, distributions of electric potentials appear at interfaces between recorded portions and unrecorded portions of the ink layer. This phenomenon is referred to as the edge effect. Adhesion of the powdered ink in the vicinity of the interface is increased. This leads to an uneven layer of ink and problems occur when attempting to adjust the thickness of the regenerated ink layer.
Accordingly, it is desirable to develop an improved method and apparatus for replenishing an ink sheet which avoids the shortcomings of the prior art.